Protection by rhynchophylline against MPTP/MPP⁺-induced neurotoxicity via regulating PI3K/Akt pathway

Isolated from Uncaria rhynchophylla (U. rhynchophylla), rhynchophylline (Rhy) has been applied for treating diseases related to central nervous system such as Parkinson's disease. Nevertheless, the molecular mechanism of the neuroprotective effect has not been well interpreted.

To investigate the effects of Rhy on MPTP/MPP + -induced neurotoxicity in C57BL/6 mice or PC12 cells and study the mechanisms involved.

The neuroprotective effect of Rhy on MPTP-induced neurotoxicity was evaluated by spontaneous motor activity test, as well as a test of rota-rod on a rat model of Parkinson's disease. The numbers of TH-positive neurons in the substantia nigra pars compacta (SNpc) was assessed by immunohistological. CCK-8, lactate dehydrogenase (LDH), reactive oxygen species (ROS), the concentration of intracellular calcium ([Ca²⁺]i) and flow cytometry analysis were performed to evaluate the pharmacological property of Rhy on 1-methyl-4-phenylpyridinium (MPP⁺) induced neurotoxicity in PC12 cells. Besides, LY294002, a PI3K inhibitor was employed to determine the underlying molecular signaling pathway revealing the effect of Rhy by western-blot analysis.

The results showed that Rhy exhibited a protective effect against the MPTP-induced decrease in tyrosine hydroxylase (TH)-positive fibers in the substantia nigra at 30 mg/kg, demonstrated by the immunohistological and behavioral outcomes. Furthermore, it has been indicated that cell viability was improved and the MPP⁺-induced apoptosis was inhibited after the treatment of Rhy at 20 μM, which were severally analyzed by the CCK-8 and the Annexin V/propidium iodide staining method. In addition, Rhy treatment attenuated MPP⁺-induced up-regulation of LDH, ([Ca²⁺]i), and the levels of ROS. Besides, it can be revealed from the Western blot assay that LY294002, as a selective Phosphatidylinositol 3-Kinase (PI3K) inhibitor, effectively inhibited the Akt phosphorylation caused by Rhy, which suggested that Rhy showed its protective property through the activated the PI3K/Akt signaling pathway. Moreover, the Rhy-induced decreases of Bax and caspase-3 as the proapoptotic markers and the increase of Bcl-2 as the antiapoptotic marker, were blocked by LY294002 in the MPP⁺-treated PC12 cells.

Rhy exerts a neuroprotective effect is partly mediated by activating the PI3K/Akt signaling pathway.